1. Field of the Invention
This invention pertains to a method of determining the porosity and the irreducible water saturation associated with a coal cleat network. These parameters are useful in determining the amount of mobile water in a coal seam. Mobile water is water-in-place in the coal seam which will have to be removed and disposed of during coal degasification operations.
2. Description of the Prior Art
Coal is the most abundant fossil fuel in the world. Its recoverable reserves amount to almost 100 quintillion BTU of energy, nearly 15 times the total energy content estimated for known reserves of petroleum. Petroleum Frontiers, vol. 3, no. 4, pages 2-3 (1986), published by Petroleum Information Corporation. People have mined coal and used it for heat for centuries. However, it is within the recent past that coal has been recognized for being the origin and source for coalbed methane, another valuable hydrocarbon fuel. "Coalbed methane" consists primarily of methane (e.g., 95%) but may also contain ethane, propane, and higher homologs. The volume of coalbed methane is estimated to be about 400 trillion standard cubic feet (SCF) of gas in place, most of it adsorbed on coal in seams buried at a depth of less than 9000 feet (ft) from the surface, and almost half of it is on coal seams buried less than 3000 ft, too deep to mine but easily penetrated by a wellbore using conventional drilling techniques. Coalbeds are, therefore, reservoirs and source rocks for a huge amount of gas which can be produced, in part, through a wellbore. Methods of recovering the gas (i.e., coal degasification methods) are shown, for example, by U.S. Pat. No. 4,471,840, U.S. Pat. No. 4,391,327 and U.S. Pat. No. 4,301,875.
Much work has been done to capture the prize. The U.S. Department of Energy and the Gas Research Institute have funded a substantial amount of research on coal degasification and the results have been published in the open literature. In addition, periodic coalbed methane symposiums are held at the University of Alabama, and elsewhere, and the results published as symposium proceedings. Many of the journal articles highlight the significance of determining the water-in-place in the coal seam which will have to be removed and disposed of during coal degasification operations. The present invention is directed to this technical problem.
Coal is a dual porosity rock consisting of micropores (matrix) and a network of natural fractures known as cleats. At discovery, the cleat network and matrix micropores in a coal seam are completely saturated with water and methane is adsorbed to the surface of coal. Reservoir pressure depletion is the only mechanism currently being employed to desorb methane from coal. When production of coalbed methane is initiated, water contained in the coal cleat network flows to the wellbore, as per Darcy's Law. This leads to a reduction in reservoir pressure which in turn desorbs methane from the coal surface. Gas production rate from a well is accordingly directly influenced by the speed with which a coal seam is de-watered. While methane migrates from the coal matrix to the cleat network by diffusion, the water contained in the coal micropores (typically 40 Angstrom or smaller pores linked by 5 Angstrom passages) remains essentially immobile due to strong capillary forces. Thus, even though most of the porosity in coal is contained within the micropores, only the cleat porosity and its irreducible water saturation are of importance to a coalbed methane project.
The present invention provides a means for determining the porosity and irreducible water saturation associated with a coal cleat network.